The present invention relates generally to a method and an arrangement for measuring the concentration of gases in a sample and, more particularly, to a method and an arrangement which optically measures the concentration of oxygen and carbon dioxide dissolved in blood.
It is known in the prior art to provide test cells filled with an indicating substance which reacts with a gas component to be measured in a sample by emitting a fluorescent-type light beam when the indicating substance is illuminated by monochromatic light. When the concentration or distribution of the gas component changes, the indicating substance changes its color characteristic and intensity. In turn this difference in intensity is measured by a light-measuring device. Fluorescent-type indicators are popular because it is relatively simple to filter the fluorescent-type component, thus making possible a high signal-to-noise ratio.
However, fluroescent-type indicators have not been successfully used to measure gases dissolved in the blood stream, because, for example, albumin in the blood interferes with the reaction of the indictor substance.
Thus, the prior art proposed various electrode techniques to measure the concentration of gases. However, such electrode-measuring techniques are possessed of many disadvantages. For example, in the measurement of the partial pressure of oxygen, the polarization-type electrodes must be constituted of very pure material which requires a great deal of costly and frequent maintenance. Moreover, the concentration distribution can only be determined on a point-by-point basis. Even if the prior art electrodes are inserted transcutaneously through the skin, then still only very slight gas quantities ever reach the electrodes. Furthermore, the measurement is strongly influenced by the inherent properties of the electrodes themselves. Since the electrodes require a large amount of gas to flow towards them so as to generate a detectable electrical current, electrode arrangements having large surface areas are impractical. Thus, oxygen concentration distributions are not readily attainable in the prior art.
With respect to the partial pressure measurement of gases other than oxygen, Stow and Randall Amer. J. Physiol. 179/678p-1954 disclose the measurement of carbondioxide with glass electrodes. However, such electrodes require measuring times of over 30 seconds which are undesirably long in most applications. Moreover, the accuracy of the measurement is substantially reduced by the presence of the required reference electrode.